1. Field of the Invention
The present invention relates to a high temperature corrosion resistant alloy, and a thermal barrier coating material, a turbine member, and a gas turbine using the high temperature corrosion resistant alloy. In particular, the present invention relates to a composition of a high temperature corrosion resistant alloy having an excellent oxidation resistance and ductility which is suitable for use in a metal bonding layer of a thermal barrier coating material.
2. Description of Related Art
Recently, as one of the energy saving countermeasures, improvements in thermal efficiency in thermal power generation has been studied. In order to improve the power generation efficiency of a gas turbine used for generating power, it is effective to increase a gas inlet temperature of a turbine, and the inlet temperature is often increased to 1,500xc2x0 C. In order to realize such a high temperature of a power generation device, it is necessary to use a heat resisting material for a stationary vane and a rotor vane of a gas turbine, or for a wall of a combustor. However, even if a turbine vane is made using a heat resisting metal, it cannot withstand such a high temperature by itself. Accordingly, as shown in FIG. 7, it is generally carried out that a metal bonding layer 102 is formed on a base material 101 made of a heat resisting metal, and a ceramic layer 103 made of an oxide ceramics is laminated on the metal bonding layer 102 using a film forming method, such as thermal spraying, to form a thermal barrier coating (TBC) in order to be protected from a high temperature. As a metal bonding layer 102, a MCrAlY alloy (where M is Co, Ni, or a combination thereof) is known, and as a ceramic layer 103, a ZrO2 type material, especially, a yttria stabilized zirconia (YSZ), which is a ZrO2 partially stabilized or completely stabilized by Y2O3, is often used due to its relatively low thermal conductivity and relatively high thermal expansion rate.
It is possible to improve the heat resistance of a base material using the thermal barrier coating mentioned above. However, due to the use of a high temperature in a gas turbine in these days, it is expected that the inlet temperature of the turbine exceeds 1,500xc2x0 C. depending on a kind of the gas turbine, and the inlet temperature of a recently developed ultra high temperature gas turbine, which is developed as one of the environmental countermeasures, may reach 1,700xc2x0 C. Also, it is considered that the temperature at the surface of the thermal barrier coating of a turbine vane reaches about 1,300xc2x0 C. Accordingly, thermal stress due to difference in linear expansion coefficient of a high temperature part, such as a turbine vane, becomes large since the difference in temperature of a heat cycle associated with the actuation of the turbine becomes large. For this reason, cracks may be generated in the metal bonding layer 102 during the operation of the turbine, and there is a danger that the cracks reach the base material 101 or the ceramic layer 103 may be separated from the bonding layer 102.
Accordingly, it is required to improve the ductility of the metal bonding layer 102 in order to prevent the generation of cracks in the metal bonding layer 102. Also, it is required to improve the corrosion resistance and oxidation resistance of the metal bonding layer 102 since it is expected that the corrosion or the oxidation of a turbine vane, etc., will significantly increase along with increases in the gas temperature due to corrosive components contained in the fuel or salinity of the air flow.
The present invention takes into consideration the above-mentioned circumstances, and has as an object of providing a high temperature corrosion resistant alloy having excellent oxidation and corrosion resistance, and ductility
Also, another object of the present invention is to provide a thermal barrier coating material with excellent exfoliation resistance including a metal bonding layer which is formed by the above-mentioned alloy.
Moreover, yet another object of the present invention is to provide a turbine member which is coated with the above-mentioned thermal barrier coating, and to provide a gas turbine including the turbine member.
The inventors of the present invention, in order to achieve the above objects, have carried out diligent studies on the composition of a MCrAlY alloy which forms a metal bonding layer, and have found that a metal bonding layer having excellent ductility and oxidation resistance can be formed by using a high temperature corrosion resistant alloy having the following composition, and completed the present invention.
That is, the high temperature corrosion resistant alloy according to an embodiment of the present invention includes 0.1-12% by weight of Co, 10-30% by weight of Cr, 4-15% by weight of Al, 0.1-5% by weight of Y, and 0.5-10% by weight of Re, and the rest is substantially formed by Ni.
A thermal barrier coating material including a metal bonding layer having excellent ductility and oxidation resistance may be made by forming the metal bonding layer on a base material using a high temperature corrosion resistant alloy having the above composition, and laminating a ceramic layer on the metal bonding layer. That is, stress applied to the ceramic layer laminated on the metal bonding layer can be reduced by the excellent ductility of the metal bonding layer, and hence, it becomes possible to prevent the ceramic layer from being separated from the metal bonding layer. Also, it becomes possible to prevent oxidation and corrosion of the base material at high temperatures due to the excellent oxidation resistance of the metal bonding layer, and a long-life thermal barrier coating material can be realized. Moreover, the metal bonding layer formed by using the high temperature corrosion resistant alloy having the above-mentioned composition has excellent affinity with stabilized zirconia which is often used for a ceramic layer, and hence, the ceramic layer may be firmly bonded so that it does not readily separate from the metal bonding layer.
Hereinafter, the function and appropriate weight range of each element contained in the high temperature corrosion resistant alloy according to an embodiment of the present invention will be explained.
Co (0.1-12% by weight):
The greater the amount of Co added, the more it increases the ductility of the high temperature corrosion resistant alloy. If the amount of Co is less than 0.1% by weight, a sufficient effect cannot be obtained. If the amount of Co is increased to exceed 12% by weight, the effect obtained will not change.
Cr (10-30% by weight):
The greater the amount of Cr added, the more it increases the oxidation resistance of the high temperature corrosion resistant alloy. If the amount of Cr is less than 10% by weight, a sufficient oxidation resistance cannot be obtained. However, if the amount of Cr is increased to exceed 30% by weight, the hardness of the resultant alloy is increased, and the ductility thereof is decreased. In addition to that, dense formation of Al2O3 is inhibited. Accordingly, it is more preferable that the added amount of Cr be in the range of 15-25% by weight from the viewpoint of a balance between the oxidation resistance and the ductility.
Al (4-15% by weight):
When the high temperature corrosion resistant alloy is used for the metal bonding layer of the thermal barrier coating, Al has the effects of densely forming Al2O3 on the surface thereof to improve the oxidation resistance of the metal bonding layer, and improving the oxidation resistance of the thermal barrier coating, for instance. If the amount of Al is less than 4% by weight, dense formation of Al2O3 will not be occur due to the generation of (Ni, Co)(Cr, Al)2O4 spinel composite oxide, and the effect of improving the oxidation resistance cannot be obtained. Also, if the amount of Al is increased to exceed 15% by weight, an intermetallic compound (Ni, Coxe2x80x94Al) phase formed by the interaction of Al with Ni and Co, which are contained in the high temperature corrosion resistant alloy, is produced increasing the hardness and decreasing the ductility of the alloy, and hence, this is not preferable. It is more preferable that the amount of Al added be in the range of 4-8% by weight since a high temperature corrosion resistant alloy having better ductility can be produced.
Y (0.1-5% by weight):
Addition of Y prevents the separation of Al2O3 scales from the surface of the metal bonding layer. However, if the amount of Y is too large, it makes the high temperature corrosion resistant alloy brittle, and decreases the thermal shock resistance. Accordingly, the upper limit of the addition is defined to be 5% by weight. Also, if the amount of Y is less than 0.1% by weight, a sufficient effect will not be obtained. It is more preferable that the amount of Y added be in the range of 0.1-1% by weight.
Re (0.5-6% by weight):
Re has an effect of increasing the density of the above-mentioned Al2O3 scales formed on the surface of the metal bonding layer, which is made using the high temperature corrosion resistant alloy, to improve the corrosion resistance of the high temperature corrosion resistant alloy. Also, Re has an effect of forming a CrRe compound in an oxidation denatured layer, which is formed directly below the Al2O3 scales, to prevent brittleness of the oxidation denatured layer, and to inhibit the growth of the Al2O3 scales so that the life of the thermal barrier coating film can be prolonged.
The above-mentioned oxidation denatured layer is formed along with a decrease in the concentration of Al in the vicinity of the metal bonding layer surface and a relative increase in the concentration of Cr and Ni. In such a Cr and Ni rich state, compounds such as NiCr2O4, and Cr2O3 which are of low density and brittle, tend to occur in the oxidation denatured layer. However, if the metal bonding layer is formed using the high temperature corrosion resistant alloy according to the embodiments of the present invention, since the Cr concentration of the above oxidation denatured layer is lowered, it becomes possible to prevent the above-mentioned low density compounds from being produced. Accordingly, it also becomes possible to prevent the thermal shock resistance of the metal bonding layer from being lowered.
If the content of Re is less than 0.5% by weight, the above effect cannot be obtained since there is almost no formation of the CrRe compounds explained above. Also, if the amount of Re is increased exceeding 10% by weight, the resulting product is hardened and the ductility thereof is decreased.
In the high temperature corrosion resistant alloy according to an embodiment of the present invention, it is preferable, in particular, that the content of Re be in the range of 0.5-6% by weight, and it is more preferable that the content of Re be in the range of 0.5-4% by weight. If the content of Re is controlled to be in the above range, it becomes possible to obtain a long-life metal bonding layer having excellent ductility in which the growth of Al2O3 scales is slow and do not readily separated from the surface of the metal bonding layer.
The high temperature corrosion resistant alloy according to an embodiment of the present invention may also include 0.01-0.7% by weigh of Hf and/or 0.01-1.5% by weight of Si.
Hf (0.01-0.7% by weight):
Similar to the above-mentioned Y, Hf has an effect of preventing the separation of Al2O3 scales from the surface of the metal bonding layer. In this manner, Hf prevents the separation of the ceramic layer, which is laminated on the metal bonding layer, to prolong the life of the thermal barrier coating material. However, if the amount of Hf added is too large, it makes the high temperature corrosion resistant alloy brittle. Accordingly, it is preferable that the upper limit of Hf be 0.7% by weight.
Si (0.01-1.5% by weight):
Si prevents the growth of Al2O3 on the surface of the metal bonding layer, and has an effect of prolonging the life of the metal bonding layer. If the amount of Si added is less than 0.01% by weight, the above effect cannot be obtained. Also, if Si is added to exceed 1.5% by weight, the high temperature corrosion resistant alloy is hardened, and the ductility thereof tends to be lowered.
The thermal barrier coating material according to an embodiment of the present invention includes a heat-resistant alloy base material; a metal bonding layer disposed on the heat-resistant alloy base material, the metal bonding layer being formed of any one of the above-mentioned high temperature corrosion resistant alloy composition, and a ceramic layer disposed on the metal bonding layer.
The thermal barrier coating material according to an embodiment of the present invention, since it includes the metal bonding layer formed by using any one of the above-mentioned high temperature corrosion resistant alloy compositions, has excellent oxidation resistance, corrosion resistance, and ductility. Accordingly, if applied to a high temperature part, it becomes possible to effectively prevent oxidation and corrosion of the part due to high temperatures, and to impart high durability to the part by preventing the generation of cracks in the metal bonding layer associated with heat cycles. Also, since the metal bonding layer formed by the high temperature corrosion resistant alloy composition according to an embodiment of the present invention has excellent affinity to not only the heat-resistant alloy which forms the base material but also the ceramic material which forms a ceramic layer, such as stabilized zirconia, it becomes possible to more firmly fix the ceramic layer, which is a thermal barrier layer, and in this point also, a thermal barrier coating material in which separation of the ceramic layer does not readily occur may be realized.
In accordance with another aspect of the present invention, in the above thermal barrier coating material, an oxidation scale layer including Al2O3 as its main constituent is formed in the metal bonding layer at the boundary between the metal bonding layer and the ceramic layer, and an oxidation denatured layer is formed in the metal bonding layer at a position below the oxidation scale layer. The content of Al of the oxidation denatured layer is lowered due to the formation of the oxidation scale layer, and the oxidation denatured layer contains precipitates including CrRe compounds its main constituent.
That is, the metal bonding layer of the thermal barrier coating material according to an embodiment of the present invention is formed by the high temperature corrosion resistant alloy composition so that dense Al2O3 scales are formed on the surface of the metal bonding layer, and so that precipitates containing CrRe compounds are produced in an oxidation denatured layer, which is generated due to the formation of the Al2O3 scales, in order to prevent the generation of low density brittle compounds, such as NiCr2O4, and Cr2O3, to realize a thermal barrier coating material having excellent oxidation and corrosion resistance. Also, since the growth of Al2O3 is inhibited due to the formation of the above CrRe compounds, it becomes possible to maintain an appropriate thickness of the Al2O3 scales for a long period of time. Accordingly, it becomes possible to provide a long-life thermal barrier coating material according to an embodiment of the present invention in the above-mentioned manner.
In yet another aspect of the present invention, in the above thermal barrier coating material, the metal bonding layer may be made into a film by a method comprising a step of thermal spraying a powder of any one of the above high temperature corrosion resistant alloy compositions, by a method comprising a step of depositing any one of the above high temperature corrosion resistant alloy compositions using an electron beam physical deposition method.
The present invention also provides a turbine member including any one of the thermal barrier coating materials. That is, a long-life turbine member having excellent oxidation and corrosion resistance in which separation of the ceramic layer does not readily occur may be provided by using the above thermal barrier coating material.
The present invention also provides a gas turbine including the above turbine member. The gas turbine which is formed by using the turbine member having excellent oxidation and corrosion resistance according to an embodiment of the present invention may be stably operated for a long period of time with high efficiency even when a gas at a high temperature is used.